I haven't followed mix panels development. I was surprised that a company which went as far as writing their own time series database apparently decided to go all in on Google cloud.
As a startup with limited resources, it's important for us to invest all our engineering strength into the things that create direct value for our business. We'd rather pay Google to manage machines and run services like Kubernetes, Spanner, Pub/Sub and others and free up the engineers to work on our core analytics platform.
* analyze every datapoint you receive
* when the dimensional cardinality is high
* you want to analyze behaviors over time (e.g. the output depends on the orders of events followed - like creating a funnel report)